1) Field of the Invention
The present invention relates to explosively actuated tools for driving a fastener such as a pin into a substrate such as concrete or steel.
2) Description of the Prior Art
Explosively actuated tools for driving a fastener such as a pin into a substrate such as concrete or steel conventionally comprise a driving piston which is driven forwardly along the barrel of the tool upon detonation of an explosive charge to drive into the substrate a fastener within the forward end of the barrel. Tools of this type conventionally comprise a safety mechanism whereby to fire the tool, the firing mechanism must be cocked by pushing the forward end of the barrel against the substrate. This causes the barrel to retract through a limited distance into the housing of the tool and this movement, in turn, cocks the tool and enables firing of the tool. This safety mechanism is designed to ensure that the tool can only be fired in its operative position pressed against the substrate and various forms of cocking mechanism responsive to the retraction of the barrel within the tool are well known per se to those skilled in this art.
In International patent application PCT/AU90/00018 (WO 90/08628) there is disclosed a tool of the type just described in which the firing pin is held in its cocked position by a rotary sear which is displaced rearwardly during cocking by pressing the forward end of the barrel against the substrate. The sear entrains the firing pin during the cocking action against the bias of the firing pin spring. The sear itself is mounted on a cocking rod which, on cocking, is displaced rearwardly against the bias of a separate compression coil spring. This coil spring not only applies a forwards axial bias to the cocking rod and sear carried thereby, but also a rotational or torsional bias in a sense to pivot the sear into engagement with the firing pin. In existing tools of this type currently in production, the cocking rod and firing pin are mounted in a housing of block-like form. The coil spring which axially and rotationally biases the cocking rod extends rearwardly from the cocking rod to be received in a blind bore at the rear end of the housing. During assembly, insertion of the rear end of the spring into the bore induces a twist in the spring to provide the torsional bias. The twist which can be achieved in this way is typically not more than about 90°. As a consequence, the coil spring must itself be sufficiently strong to provide the required torsional bias just with a twist of approximately 90°.
After the tool has been fired, the driving piston is within the forward end of the barrel and appropriate action must be taken to reset the piston into a rear position within the barrel in preparation for the next driving stoke. In many existing tools, resetting is accomplished by a manual action involving the operator drawing the barrel forwardly from the housing of the tool while the piston is restrained so that the piston then lies within a rear part of the barrel which is then retracted manually back into the housing. However, tools with semi-automatic or fully automatic resetting systems are now widely produced. One form of semi-automatic resetting system involves resetting of the piston when cocking the tool and an example of this is described in International patent application PCT/AU99/01097 (WO 00/35638). This type of resetting system necessarily requires the application of a greater force needed to cock the tool as one or more additional springs are incorporated as part of the resetting mechanism and are loaded during cocking. Thus, cocking must take place against the combined forces of the spring(s) of the resetting mechanism, the firing pin spring, and the cocking rod/sear spring.